The present invention relates to a cathode ray tube, such as a color image receiving tube.
For example, a color cathode ray tube generally comprises a vacuum envelope which includes a glass face panel having a substantially rectangular display portion, a glass funnel connected to the face panel, and a cylindrical glass neck connected to the funnel. An electronic gun for emitting three electron beams is arranged within the neck. A deflection yoke is fitted on the periphery of the neck and the periphery of a part of the funnel. The funnel has a small-diameter portion, a so-called yoke attachment portion, which extends from a joint between the funnel and the neck to a portion where an end of the deflection yoke is located.
A phosphor screen is formed on the inner surface of the face panel. The screen comprises three-color phosphor dotted or stripe layers which emit blue, green and red light, respectively. Within the vacuum envelope, a shadow mask having a number of electron beam passage apertures faces the phosphor screen.
The electron beams emitted from the electron gun are deflected in horizontal and vertical directions by horizontal and vertical deflection magnetic fields generated by the deflection yoke, so that the phosphor screen is scanned by the electron beams horizontally and vertically through the shadow mask, thereby displaying color images.
As a type of the cathode ray tubes, a color cathode ray tube of self-convergence in-line type has been widely put into practical use. The cathode ray tube of this type has an in-line electron gun, which emits three electron beams arranged in line on the same horizontal plane. The three electron beams emitted from the electron gun are deflected by a horizontal deflection magnetic field, shaped like a pin cushion, and a vertical deflection magnetic field, shaped like a barrel. Thus, the three electron beams in line are converged at a point in any portion of the screen without using special correcting means.
In the cathode ray tube as described above, the deflection yoke consumes a great amount of power. To reduce the power consumption of the cathode ray tube, therefore, it is important to reduce the power consumption of the deflection yoke. The deflection power is increased in the following situations: when the cathode voltage for accelerating the electron beams is increased to increase the screen luminance at last; and when the deflection frequency is increased to use the cathode ray tube in OA equipment, e.g., an HD (High Definition) device or PC (Personal Computer).
Regarding a type of the OA equipment, such as a PC, which is operated by the operator being close to the cathode ray tube, control over leakage magnetic field (magnetic-field leaked from the deflection yoke to the outside of the cathode ray tube) has been tightened. Conventionally, to reduce the leakage magnetic field, a compensation coil is added to the cathode ray tube. However, when the compensation coil is added, the power consumption of the PC is increased accordingly.
In general, to reduce the deflection power and the leakage magnetic field, it is preferable to decrease the diameter of the neck of the cathode ray tube and the outer diameter of the yoke attachment portion of the funnel, to which the deflection yoke is attached, so that the interaction space of the deflection magnetic field can be reduced and the deflection magnetic field can efficiently be exerted on the electron beams.
However, in the cathode ray tube, the electron beams pass near the inner surface of the yoke attachment portion of the funnel. Therefore, if the neck diameter or the diameter of the yoke attachment portion is reduced, the electron beams directed to the corner portions of the phosphor screen at the maximum deflection angle will collide against the inner surface of the yoke attachment portion. As a result, the electron beams cannot be applied to some portions of the phosphor screen. Therefore, in the conventional cathode ray tube, it is difficult to sufficiently reduce the neck diameter and the outer diameter of the yoke attachment portion in order to reduce the deflection power.
Further, if the electron beams continuously collide against the inner surface of the yoke attachment portion of the funnel, the temperature of that portion rises so high that the glass forming the funnel will be melted. In this case, the vacuum envelope may be imploded.
Jpn. Pat. Appln. KOKOKU Publication No. 48-34349 (corresponding to U.S. Pat. No. 3,731,129) discloses means for solving the above problems. According to the disclosure of the publication, the yoke attachment portion of the funnel to which the deflection yoke is attached has a substantially pyramidal shape. In other words, the cross section of the yoke attachment portion of the funnel is gradually changed from a circle to a rectangle from the neck toward the face panel. This structure is based on the idea that the electron beams pass a substantially rectangular region inside the yoke attachment portion, when drawing a rectangular raster on the phosphor screen.
If the yoke attachment portion of the funnel is shaped like a pyramid, the diameters of the portion along the major axis (horizontal axis: H axis) and the minor axis (vertical axis: V axis) can be reduced. For this reason, the horizontal and vertical deflection coils of the deflection yoke can be close to the electron beams, thereby efficiently deflecting the beams. As a result, the deflection power can be saved.
However, as the cross section of the yoke attachment portion of the funnel becomes nearly rectangular to effectively reduce the deflection power as described above, side end portions of the yoke attachment portion along the horizontal and vertical axes become flat. Therefore, the side end portions are distorted toward the axis of the tube by the load of the atmospheric pressure. As a result, compressive stress .sigma.H and .sigma.V are generated on the outer surface of the yoke attachment portion near the side end portions along the horizontal and vertical axes, and great tensile stress .sigma.O is generated on the outer surface of the yoke attachment portion near the ends of the diagonal axes. Thus, the strength against the atmospheric pressure of the vacuum envelope is reduced and the safety is impaired.
Further, at present, there is a great demand for prevention of a reflection of external light on the face panel and high visibility of an image. For this purpose, it is necessary that the face panel be flat. However, if the face panel is flat, the strength of the vacuum envelope is reduced. Therefore, when the conventional funnel having a pyramidal yoke attachment portion is used, it is difficult to maintain the strength of the panel necessary for safety.
For the reasons stated above, the conventional cathode ray tube has problem that the yoke attachment portion of the funnel cannot be so rectangular as to sufficiently reduce the deflection power or that the rectangular yoke attachment portion cannot be applied to a flat face panel. Therefore, according to the conventional art, although the yoke attachment portion of the funnel can be shaped like a pyramid, it cannot reduce the deflection power while maintaining sufficient strength against the atmospheric pressure.